This invention relates to vehicle suspension dampers for use in controlled damping vehicle suspension systems. Such systems incorporate shock or strut devices capable of varying their damping characteristics in response to an electronic control signal. This signal is typically generated in an electronic control in response to one or more suspension related input signals. In a closed loop suspension control, an important input signal indicates the velocity of movement between the vehicle sprung mass, or body, and the vehicle unsprung mass, or wheel assembly.
The prior art includes publications describing systems in which the vehicle suspension at a wheel includes a suspension relative position sensor such as an LVDT. The position signal from such a sensor may be differentiated to provide a relative velocity signal; however, the process requires additional electronic circuitry with difficult design requirements.
The prior art also includes relative velocity sensors incorporated in suspension components such as dampers. For example, the patent to Herberg et al 5,009,450 discloses such a sensor incorporated in a vehicle shock absorber of the type having a cylinder attached to one of the sprung and unsprung masses and a piston in the cylinder attached through a rod extending out of the cylinder to the other of the sprung and unsprung masses. The rod further carries a dust tube which extends over a substantial portion of the cylinder. An axially polarized annular magnet is attached to but magnetically spaced from the top of the cylinder and is further magnetically spaced from the piston rod; and a sensor winding is distributed axially along the inside of the dust tube, which is made of a non-magnetic material. Vertical motion between the sprung and unsprung masses causes similar axial motion between the dust tube and cylinder and moves the magnet axially along the sensor winding.
However, the Herberg et al sensor provides a low output through the middle of its range and loses its linearity near the ends of its range. The variation of flux linkage with position is very flat and actually reverses direction near one end. In addition, with its uncontrolled flux paths, it is highly susceptible to the presence of nearby magnetic objects, such as other suspension members, which may distort the flux paths. Thus, the output voltage is low and is not a dependably accurate indication of the relative velocity of the sprung and unsprung masses of the vehicle.